2011-09-13 17:44:24 +00:00
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
< html xmlns = "http://www.w3.org/1999/xhtml" >
< head >
< meta http-equiv = "Content-Type" content = "text/html; charset=UTF-8" / >
< title > Db::exists()< / title >
< link rel = "stylesheet" href = "apiReference.css" type = "text/css" / >
< meta name = "generator" content = "DocBook XSL Stylesheets V1.73.2" / >
< link rel = "start" href = "index.html" title = "Berkeley DB C++ API Reference" / >
< link rel = "up" href = "db.html" title = "Chapter / >
< link rel = "prev" href = "dberr.html" title = "Db::err()" / >
< link rel = "next" href = "dbfd.html" title = "Db::fd()" / >
< / head >
< body >
< div xmlns = "" class = "navheader" >
< div class = "libver" >
< p > Library Version 11.2.5.2< / p >
< / div >
< table width = "100%" summary = "Navigation header" >
< tr >
< th colspan = "3" align = "center" > Db::exists()< / th >
< / tr >
< tr >
< td width = "20%" align = "left" > < a accesskey = "p" href = "dberr.html" > Prev< / a > < / td >
< th width = "60%" align = "center" > Chapter
The Db Handle
< / th >
< td width = "20%" align = "right" > < a accesskey = "n" href = "dbfd.html" > Next< / a > < / td >
< / tr >
< / table >
< hr / >
< / div >
< div class = "sect1" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
< h2 class = "title" style = "clear: both" > < a id = "dbexists" > < / a > Db::exists()< / h2 >
< / div >
< / div >
< / div >
< pre class = "programlisting" > #include < db_cxx.h>
int
Db::exists(DbTxn *txnid, Dbt *key, u_int32_t flags);< / pre >
< p >
The < code class = "methodname" > Db::exists()< / code > method returns whether the specified key appears in the
database.
< / p >
< p >
The < code class = "methodname" > Db::exists()< / code > method will return < a href = "../../programmer_reference/program_errorret.html#program_errorret.DB_NOTFOUND" class = "olink" > DB_NOTFOUND< / a > if the specified key
is not in the database. The < code class = "methodname" > Db::exists()< / code > method will return
< a href = "../../programmer_reference/program_errorret.html#program_errorret.DB_KEYEMPTY" class = "olink" > DB_KEYEMPTY< / a > if the database
is a Queue or Recno database and the specified key exists, but was never explicitly created
by the application or was later deleted.
< / p >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2011-12-20 00:07:10 +00:00
< h3 class = "title" > < a id = "id3561558" > < / a > Parameters< / h3 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< div class = "sect3" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2011-12-20 00:07:10 +00:00
< h4 class = "title" > < a id = "id3561903" > < / a > txnid< / h4 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< p >
If the operation is part of an application-specified transaction, the < span class = "bold" > < strong > txnid< / strong > < / span > parameter is a transaction handle returned from
< a class = "xref" href = "txnbegin.html" title = "DbEnv::txn_begin()" > DbEnv::txn_begin()< / a > ; if the operation is part of a
Berkeley DB Concurrent Data Store group, the < span class = "bold" > < strong > txnid< / strong > < / span >
parameter is a handle returned from
< a class = "xref" href = "envcdsgroup_begin.html" title = "DbEnv::cdsgroup_begin()" > DbEnv::cdsgroup_begin()< / a > ; otherwise NULL.
If no transaction handle is specified, but the
operation occurs in a transactional database, the operation will be
implicitly transaction protected.
< / p >
< / div >
< div class = "sect3" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2011-12-20 00:07:10 +00:00
< h4 class = "title" > < a id = "id3561915" > < / a > key< / h4 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< p >
The key < a class = "link" href = "dbt.html" title = "Chapter > Dbt< / a > operated on.
< / p >
< / div >
< div class = "sect3" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2011-12-20 00:07:10 +00:00
< h4 class = "title" > < a id = "id3562046" > < / a > flags< / h4 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< p >
The < span class = "bold" > < strong > flags< / strong > < / span > parameter must be set to zero or
by bitwise inclusively < span class = "bold" > < strong > OR< / strong > < / span > 'ing together one
or more of the following values:
< / p >
< div class = "itemizedlist" >
< ul type = "disc" >
< li >
< p > < a id = "exists_DB_READ_COMMITTED" > < / a >
< code class = "literal" > DB_READ_COMMITTED< / code >
< / p >
< p >
Configure a transactional read operation to have degree 2 isolation
(the read is not repeatable).
< / p >
< / li >
< li >
< p > < a id = "exists_DB_READ_UNCOMMITTED" > < / a >
< code class = "literal" > DB_READ_UNCOMMITTED< / code >
< / p >
< p >
Configure a transactional read operation to have degree 1 isolation,
reading modified but not yet committed data. Silently ignored if the
< a class = "link" href = "dbopen.html#dbopen_DB_READ_UNCOMMITTED" > DB_READ_UNCOMMITTED< / a >
flag was not specified when the underlying database was opened.
< / p >
< / li >
< li >
< p > < a id = "exists_DB_RMW" > < / a >
< code class = "literal" > DB_RMW< / code >
< / p >
< p >
Acquire write locks instead of read locks when doing the read, if locking is
configured. Setting this flag can eliminate deadlock during a read-modify-write
cycle by acquiring the write lock during the read part of the cycle so that
another thread of control acquiring a read lock for the same item, in its own
read-modify-write cycle, will not result in deadlock.
< / p >
< p >
Because the < code class = "methodname" > Db::exists()< / code > method will not hold locks
across Berkeley DB calls in non-transactional operations, the
< a class = "link" href = "dbcget.html#dbcget_DB_RMW" > DB_RMW< / a > flag to the
< code class = "methodname" > Db::exists()< / code > call is meaningful only in the presence
of transactions.
< / p >
< / li >
< / ul >
< / div >
< / div >
< / div >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2011-12-20 00:07:10 +00:00
< h3 class = "title" > < a id = "id3562322" > < / a > Class< / h3 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< p >
< a class = "link" href = "db.html" title = "Chapter > Db< / a >
< / p >
< / div >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2011-12-20 00:07:10 +00:00
< h3 class = "title" > < a id = "id3562195" > < / a > See Also< / h3 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< p >
< a class = "xref" href = "db.html#dblist" title = "Database and Related Methods" > Database and Related Methods< / a >
< / p >
< / div >
< / div >
< div class = "navfooter" >
< hr / >
< table width = "100%" summary = "Navigation footer" >
< tr >
< td width = "40%" align = "left" > < a accesskey = "p" href = "dberr.html" > Prev< / a > < / td >
< td width = "20%" align = "center" >
< a accesskey = "u" href = "db.html" > Up< / a >
< / td >
< td width = "40%" align = "right" > < a accesskey = "n" href = "dbfd.html" > Next< / a > < / td >
< / tr >
< tr >
< td width = "40%" align = "left" valign = "top" > Db::err() < / td >
< td width = "20%" align = "center" >
< a accesskey = "h" href = "index.html" > Home< / a >
< / td >
< td width = "40%" align = "right" valign = "top" > < / td >
< / tr >
< / table >
< / div >
< / body >
< / html >
